Wireless telephones, such as mobile/cellular telephones, cordless telephones, and other consumer audio devices, such as mp3 players, are in widespread use. Performance of such devices with respect to intelligibility can be improved by providing noise canceling using a microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal into the output of the device to cancel the ambient acoustic events.
In a traditional hybrid adaptive noise cancellation system that includes both feedforward anti-noise and feedback anti-noise, an error microphone is used to generate an error microphone signal that measures a combined acoustic pressure at an acoustic transducer (e.g., loudspeaker) including playback of a source audio signal and ambient sounds. The error microphone signal is used to generate feedback anti-noise as well as adapt a feedforward adaptive filter for generating feedforward anti-noise from a reference microphone signal configured to measure ambient sounds.
In generating the feedback anti-noise, it is critical that the feedback noise cancelling system cancel only ambient noise at the error microphone, but not the playback signal. Accordingly, a feedback adaptive noise cancellation system will often generate a playback corrected error signal equal to the error microphone signal that is typically reduced by a filtered version of the source audio signal, wherein the filter estimates the secondary path, which is the electro-acoustic path of the source audio signal through an acoustic transducer. If modeled correctly, the playback corrected error signal will be approximately equal to the ambient noise level present at the acoustic transducer.
In traditional approaches, the secondary path is estimated using offline testing and characterization, on the assumption that the secondary path does not significantly change from user to user. However, in actual application, the acoustic environment around an audio device can change dramatically, depending on the sources of noise that are present, the position of the device itself, and the physical characteristics of the user, and it may be desirable to adapt noise cancellation to take into account such environmental changes.